Concerns over high oil prices, security of supply and global warming have raised the demand for renewable energy. Renewable energy is energy produced from plant derived biomass. Renewable energy applications such as fuel ethanol are seen as a valuable contribution to the reduction in fossil fuel consumption. Public policies have supported the creation of a fuel ethanol industry largely based on the use of corn as a feedstock. The production of fuel ethanol helps to stabilize farm income and reduces farm subsidies. However, as demand increases for fuel ethanol, additional feedstocks such as lignocellulosic biomass are under consideration (1-3).
Fuel ethanol is created by the fermentation of starch derived sugars. The ethanol is distilled and dehydrated to create a high-octane, water-free gasoline substitute. Fuel ethanol is blended with gasoline to produce a hybrid fuel, which has environmental advantages when compared to gasoline alone, and can be used in gasoline-powered vehicles manufactured since the 1980's. Most gasoline-powered vehicles can run on a blend consisting of gasoline and up to 10 percent ethanol, known as “E-10” (4-10).
While corn is the major raw material for producing ethanol in North America, the dominant ethanol feedstock in warmer regions is sugarcane. It is already apparent that large-scale use of ethanol for fuel will require new technologies that will allow the industry to expand its feedstock options to include cellulose (4-10).
Cellulosic ethanol is manufactured from lignocellulosic biomass. Lignocellulosic biomass may be grouped into four main categories: (1) wood residues (including sawmill and paper mill discards), (2) municipal paper waste, (3) agricultural wastes (including corn stover, corn cobs and sugarcane bagasse), and (4) dedicated energy crops which are mostly composed of fast growing tall, woody grasses such as switch grass and Miscanthus (8-11).
Lignocellulosic biomass is composed of three primary polymers that make up plant cell walls: Cellulose, hemicellulose and lignin. Cellulose is a polymer of D-glucose. Hemicellulose contains two different polymers i.e. xylan, a polymer of xylose and glucomannan, a polymer of glucose and mannose. Lignin is a polymer of guaiacylpropane- and syringylpropane units (12-14).
Cellulose fibers are locked into a rigid structure of hemicellulose and lignin. Lignin and hemicelluloses form chemically linked complexes that bind water soluble hemicelluloses into a three dimensional array, cemented together by lignin. Lignin covers the cellulose microfibrils and protects them from enzymatic and chemical degradation. These polymers provide plant cell walls with strength and resistance to degradation, which makes lignocellulosic biomass a challenge to use as a substrate for biofuel production. Variation in the content or organization of these polymers significantly affects the overall steps of cellulosic ethanol production (12-14).
Cellulose or β-1-4-glucan is a linear polysaccharide polymer of glucose made of cellobiose units (12, 13). The cellulose chains are packed by hydrogen bonds in microfibrils (14). These fibrils are attached to each other by hemicelluloses, amorphous polymers of different sugars and covered by lignin.
Hemicellulose is a physical barrier which surrounds the cellulose fibers and protects cellulose against degradation. There is evidence that hemicellulose, containing xylose polymers (xylan), limits the activity of cellulolytic enzymes, thereby lowering cellulose to glucose conversion rates. (11-14). Thus for the production of fermentable sugars and ethanol, it is desirable to submit to the enzymatic hydrolysis a highly reactive cellulose low in xylan.
Lignin is a very complex molecule constructed of phenylpropane units linked in a three dimensional structure which is particularly difficult to biodegrade. Lignin is the most recalcitrant component of the plant cell wall. There are chemical bonds between lignin, hemicellulose and cellulose polymers. There is evidence that the higher the proportion of lignin, the higher the resistance to chemical and biological hydrolysis. Lignin and some soluble lignin derivatives inhibit enzymatic hydrolysis and fermentation processes (14, 15). Thus, it is desirable to use a lignocellulosic feedstock which is low in lignin.
The lignin content of bagasse is variable and ranges from low (10%) to high (25% by weight on a dry matter basis). The low lignin content of some bagasse residues makes this waste product a good biomass feedstock for the production of ethanol whereas high lignin bagasse is more suitable for cogeneration applications, in which bagasse is used as a fuel source to provide both heat energy, used in the mill, and electricity, which is typically sold on to the consumer electricity grid.
Published work on the various processes for the production of fermentable sugars from cellulosic biomass shows the existence of an inverse relationship between lignin content and the efficiency of enzymatic hydrolysis of sugar based polymers (16). Lignocellulosic microfibrils are associated in the form of macrofibrils. This complicated structure and the presence of lignin provides plant cell walls with strength and resistance to degradation, which also makes these materials a challenge to use as substrates for the production of biofuel and bioproducts. Thus, pretreatment is necessary to produce highly reactive cellulose that reacts well with catalysts such as enzymes (17).
Purified cellulose and lignin-free xylo-oligosaccharides are valuable for many purposes. Specifically, reactive cellulose extracted from biomass with low lignin content may be easily hydrolyzed to fermentable sugar monomers and then fermented to ethanol and other biofuels. Lignin-free xylo-oligosaccharides extracted from the hemicellulose fraction are valuable and may be easily used in the preparation of prebiotic substances for food and pharmaceutical applications.
The best method and conditions of pretreatment will vary and depend greatly on the type of lignocellulosic starting material used (18, 19). Pretreatment configuration and operating conditions must be adjusted with respect to the content or organization of lignocellulosic polymers in the starting material, to attain optimal conversion of cellulose to fermentable sugars (11). The cellulose-to-lignin ratio is the main factor. Other parameters to consider are the content of hemicellulose, degree of acetylation of hemicellulose, cellulose-accessible surface area, degree of polymerization and crystallinity (11).
An effective pretreatment should meet the following requirements: (a) production of reactive cellulosic fiber for enzymatic attack, (b) avoidance of cellulose and hemicelluloses destruction, and (c) avoidance of the formation of possible inhibitors for hydrolytic enzymes and fermenting microorganisms (17-20).
Several methods have been investigated for the pretreatment of lignocellulosic materials to produce reactive cellulose. These methods are classified into physical pretreatments, biological pretreatments and physicochemical pretreatments (21-22).
The prior art teaches that physical and biological pretreatments are not suitable for industrial applications. Physical methods such as milling, irradiation and extrusion are highly energy demanding and produce low grade cellulose. Also, the rates of known biological treatments are very low (11, 23-25).
Pretreatments that combine both chemical and physical processes are referred to as physicochemical processes (26). These methods are among the most effective and include the most promising processes for industrial applications. Hemicellulose hydrolysis is often nearly complete. As cellulose surface area increases, a decrease in cellulose degree of polymerization and crystallinity occurs. These changes greatly increase overall cellulose reactivity. Treatment rates are usually rapid (16-26). These pretreatment methods usually employ hydrolytic techniques using acids (hemicellulose hydrolysis) and alkalis for lignin removal (27-39).
The steam explosion process is well documented. Batch and continuous processes have been tested at laboratory and pilot scale by several research groups and companies (40-44). In steam explosion pretreatment, biomass is treated at high pressure, and high temperatures under acidic conditions i.e. 160° C. to 260° C. for 1 min to 20 min, at pH values<pH 4.0 (21, 17-23). The pressure of the pretreated biomass is suddenly reduced, which makes the materials undergo an explosive decompression leading to defibrization of the lignocellulosic fibers (45-47).
Steam explosion pretreatment is not very effective in dissolving lignin, but it does disrupt the lignin structure and increases the cellulose susceptibility to enzymatic hydrolysis. Steam explosion pretreatment generally results in extensive hemicellulose breakdown and, to a certain extent, to the degradation of xylose and glucose (40-44).
Steam explosion pretreatment has been successfully applied to a wide range of lignocellulosic biomasses. Acetic acid, sulfuric acid or sulfur dioxide are the most commonly used catalysts (40-44). Dilute acid- or sulfur dioxide-catalyzed steam explosion pretreatments refer to the use of 0.1-1.0% diluted sulfuric acid or 0.5-4.0% sulfur dioxide (45-47).
In acid catalyzed pretreatment processes catalysts must be recycled or the prehydrolysate stream obtained must be diluted in order to reduce the concentrations of toxic and inhibitory compounds to an acceptable level with respect to cellulolytic enzymes and fermenting organisms. As a result, large amounts of water are required prior to the enzymatic hydrolysis step. This results not only in increased capital equipment cost (tankage) but also in increased operating cost by limiting the final ethanol titer, due to the dilution factor applied to the stream of biomass prehydrolysate, which then dramatically increases energy requirements and cost of distillation (11, 20, 45-47).
In the autohydrolysis process, no added acid is required to reach pH values below 4.0. Acetic acid is released during the breakdown of acetylated hemicellulose resulting from the high pressure steam applied to the biomass during the cooking stage. The degree of hemicellulose acetylation is variable among different sources of biomass (11, 41, 48). The hemicellulose content of bagasse is high. Much of the hemicellulose is acetylated, which means the breakdown and solubilization of the hemicellulose, which occurs during pretreatment, leads to the formation of acetic acid.
The presence of acetic acid reduces the need for acid catalysts, which is beneficial to the pretreatment process and resulting downstream processing. However, acetic acid is a powerful inhibitor of both the hydrolysis and the glucose fermentation process. Acetic acid remains in the pretreated biomass and carries through to the hydrolysis and fermentation steps. A process is desired that includes a pretreatment step carried out at a pH values <pH 4.0 to maximize hemicellulose solubilization. However, after steam pretreatment, acetic acid and pre-treatment degradation products must be removed to enhance the digestibility of the cellulose in the enzymatic hydrolysis step and to enable a more rapid and complete conversion of glucose to ethanol in the fermentation step.
Pretreatment of lignocellulosic biomass is projected to be the single, most expensive processing step, representing about 20% of the total cost. Capital-intensiveness of lignocellulosic biomass pretreatment is a problem (49). Thus, a process is desired for efficient fractionation of lignocellulosic biomass into multiple streams that (i) contain value-added compounds and (ii) may significantly improve the overall economics of a biofuel production facility.
The growing commercial importance of xylo-oligosaccharides, non digestible sugar oligomers made up of xylose units, is based on their beneficial health properties; particularly the prebiotic activity and makes them good candidates as high value added bioproducts. Xylo-oligosaccharide mixtures from auto-hydrolysis of various agricultural residues exhibit a great prebiotic potential similar to commercially available xylo-oligosaccharide products (50-52).
As is apparent from the above discussion of known approaches, improving the overall ethanol yield and reducing enzyme usage or hydrolysis time are generally linked to increased operating costs. The increased costs may outweigh the value of the increased ethanol yield, rendering existing methods economically unacceptable.